Association of female sex and positive rheumatoid factor with low serum infliximab and anti-drug antibodies, related to treatment failure in early rheumatoid arthritis: results from the SWEFOT trial population

Objective: Infliximab-treated patients with rheumatoid arthritis (RA) may respond insufficiently due to low serum infliximab (sIFX) levels, caused by anti-drug antibodies (ADAs). However, monitoring of sIFX and ADAs is not routinely implemented, and levels for optimal outcome have not been validated. We searched for predictors for sIFX 0.2 μg/mL]. The primary and secondary outcome measures were low disease activity [LDA = 28-joint Disease Activity Score (DAS28) ≤ 3.2] and remission (DAS28 7.0 μg/mL showed a dose–response association with LDA (30%, 64%, 67%, and 79%, respectively, p = 0.008) and remission (10%, 45%, 39%, and 66%, p = 0.004) at trial cessation (21 months). Female patients had sIFX 0.2 μg/mL, of whom three were ADA positive at other time-points, all with high DAS28 at follow-up. Conclusion: In early RA patients receiving IFX as a second-line agent, sIFX